1. Field of the Invention
The present invention relates to a quartz sensor and a sensing device capable of measuring a sensing target in a liquid phase with high reliability.
2. Description of the Related Art
As a sensing sensor that senses and measures a small amount of substances, there is utilized a quartz sensor that uses, for instance, a quartz resonator. The quartz sensor is a sensor having an absorbing layer formed on a surface of a quartz plate thereof and measuring a concentration of a sensing target by utilizing a characteristic such that when the sensing target adheres to a surface of the absorbing layer, a natural frequency of the crystalline quartz is changed by the amount of weight of the adhered sensing target through a mass addition effect. As the absorbing layer, antibodies such as, for instance, proteins are used, and by utilizing an antibody-antigen reaction, an absorption (capture through reaction) of antigens in sample solution, which is, for instance, blood, is conducted.
As the quartz resonator used in the quartz sensor, there is known a quartz resonator called vertical field excitation type in which electrodes are formed on a front surface and a rear surface of an AT-cut quartz plate, as disclosed in Patent Document 1, for instance, and in this case, an absorbing layer is formed on the electrode on the front surface side. This type of quartz resonator, which is, for instance, a quartz resonator of 9 MHz has a series resistance of an equivalent circuit of about 10Ω, for example, in a vapor phase, but, it has the series resistance of about 200 to 300Ω in a liquid phase, which is, for instance, pure water. In the quartz sensor including the quartz resonator, the series resistance is changed due to a viscosity of the solution and, since the series resistance in the liquid phase is small, the amount of change due to the viscosity becomes large, which affects the change in resonant frequency when an oscillation is applied, resulting in that the reliability of measured results is lowered. In order to prevent the resonant frequency from being affected by the amount of change in the series resistance or to reduce the amount of change to a negligible level, it is only required to structure the quartz sensor so that the series resistance of the quartz resonator takes a large value, which is, for instance, a value of 3 kΩ.
Meanwhile, the quartz resonator of the vertical field excitation type has a Q value (f0/Δf: f0 is a resonant frequency, and Δf is a frequency band (width of a resonance curve) corresponding to 1/√2 of a maximum value of a resonant current in the resonance curve) in the liquid phase which is lower than that in the vapor phase, in which the Q value in the atmosphere is about 60000, but, it decreases to about 2000 in the water, for instance. Generally, when the Q value is low, the stability of the quartz resonator is bad and an electronic noise is large. Therefore, it can be said that there exist such problems also when the measurement is conducted in the liquid phase using the quartz resonator, and accordingly, there is desired a technique with which a higher reliability can be secured.    [Patent Document 1] Japanese Patent Application Laid-open No. 2006-78181